Receptor tyrosine kinases (RTK's) are a sub-family of protein kinases that play a critical role in cell signalling and are involved in a variety of cancer related processes including cell proliferation, survival, angiogenesis and metastasis. Currently up to 100 different RTK's including tropomyosin-related kinases (Trk's) have been identified.
Trk's are the high affinity receptors activated by a group of soluble growth factors called neurotrophins (NT). The Trk receptor family has three members—TrkA, TrkB and TrkC. Among the NTs there are (i) nerve growth factor (NGF) which activates TrkA, (ii) brain-derived growth factor (BDNF) and NT-4/5 which activate TrkB and (iii) NT3 which activates TrkC. Each Trk receptor contains an extra-cellular domain (ligand binding), a trans-membrane region and an intra-cellular domain (including kinase domain). Upon binding of the ligand, the kinase catalyzes auto-phosphorylation and triggers downstream signal transduction pathways.
Trk's are widely expressed in neuronal tissue during its development where Trk's are critical for the maintenance and survival of these cells. A post-embryonic role for the Trk/neurotrophin axis (or pathway), however, remains in question. There are reports showing that Trk's play important role in both development and function of the nervous system (Patapoutian, A. et al Current Opinion in Neurobiology, 2001, 11, 272-280).
In the past decade, a considerable number of literature documentations linking Trk signalling with cancer have published. For example, while Trk's are expressed at low levels outside the nervous system in the adult, Trk expression is increased in late stage prostate cancers. Both normal prostate tissue and androgen-dependent prostate tumors express low levels of Trk A and undetectable levels of Trk B and C. However, all isoforms of Trk receptors as well as their cognate ligands are up-regulated in late stage, androgen-independent prostate cancer. There is additional evidence that these late stage prostate cancer cells become dependent on the Trk/neurotrophin axis for their survival. Therefore, Trk inhibitors may yield a class of apoptosis-inducing agents specific for androgen-independent prostate cancer (Weeraratna, A. T. et al The Prostate, 2000, 45, 140-148).
Furthermore, the literature also shows that over-expression, activation, amplification and/or mutation of Trk's are associated with secretory breast carcinoma (Cancer Cell, 2002, 2, 367-376), colorectal cancer (Bardelli et al Science, 2003, 300, 949-949) and ovarian cancer (Davidson, B. et al Clinical Cancer Research, 2003, 9, 2248-2259).
There are a few reports of selective Trk tyrosine kinase inhibitors. Cephalon described CEP-751, CEP-701 (George, D. et al Cancer Research, 1999, 59, 2395-2341) and other indolocarbazole analogues (WO0114380) as Trk inhibitors. It was shown that CEP-701 and/or CEP751, when combined with surgically or chemically induced androgen ablation, offered better efficacy compared with mono-therapy alone. GlaxoSmithKline disclosed certain oxindole compounds as Trk A inhibitors in WO0220479 and WO0220513. Recently, Japan Tobacco reported pyrazolyl condensed cyclic compounds as Trk inhibitors (JP2003231687A). Pfizer also recently published certain isothiazole Trk A inhibitors (Bioorg. Med. Chem. Lett. 2006, 16, 3444-3448).
In addition to the above, Vertex Pharmaceuticals have described pyrazole compounds as inhibitors of GSK3, Aurora, etc. in WO0250065, WO0262789, WO03027111 and WO200437814; and AstraZeneca have reported pyrazole compounds as inhibitors against IGF-1 receptor kinase (WO0348133). AstraZeneca have also reported Trk inhibitors in International Applications WO 2005/049033, WO 2005/103010, WO 2006/082392, WO 2006/087530, and WO 2006/087538.
Another such family of RTK's is the JAK family. The JAK (Janus-associated kinase)/STAT (signal transducers and activators or transcription) signalling pathway is involved in a variety of hyperproliferative and cancer related processes including cell-cycle progression, apoptosis, angiogenesis, invasion, metastasis and evasion of the immune system (Haura et al., Nature Clinical Practice Oncology, 2005, 2(6), 315-324; Verna et al., Cancer and Metastasis Reviews, 2003, 22, 423-434).
The JAK family consists of four non-receptor tyrosine kinases Tyk2, JAK1, JAK2, and JAK3, which play a critical role in cytokine- and growth factor mediated signal transduction. Cytokine and/or growth factor binding to cell-surface receptor(s), promotes receptor dimerization and facilitates activation of receptor-associated JAK by autophosphorylation. Activated JAK phosphorylates the receptor, creating docking sites for SH2 domain-containing signalling proteins, in particular the STAT family of proteins (STAT1, 2, 3, 4, 5a, 5b and 6). Receptor-bound STATs are themselves phosphorylated by JAKs, promoting their dissociation from the receptor, and subsequent dimerization and translocation to the nucleus. Once in the nucleus, the STATs bind DNA and cooperate with other transcription factors to regulate expression of a number of genes including, but not limited to, genes encoding apoptosis inhibitors (e.g. Bcl-XL, Mcl-1) and cell cycle regulators (e.g. Cyclin D1/D2, c-myc) (Haura et al., Nature Clinical Practice Oncology, 2005, 2(6), 315-324; Verna et al., Cancer and Metastasis Reviews, 2003, 22, 423-434).
Over the past decade, a considerable amount of scientific literature linking constitutive JAK and/or STAT signalling with hyperproliferative disorders and cancer has been published. Constitutive activation of the STAT family, in particular STAT3 and STATS, has been detected in a wide range of cancers and hyperproliferative disorders (Haura et al., Nature Clinical Practice Oncology, 2005, 2(6), 315-324). Furthermore, aberrant activation of the JAK/STAT pathway provides an important proliferative and/or anti-apoptotic drive downstream of many kinases (e.g. Flt3, EGFR) whose constitutive activation have been implicated as key drivers in a variety of cancers and hyperproliferative disorders (Tibes et al., Annu Rev Pharmacol Toxicol 2550, 45, 357-384; Choudhary et al., International Journal of Hematology 2005, 82(2), 93-99; Sordella et al., Science 2004, 305, 1163-1167). In addition, impairment of negative regulatory proteins, such as the suppressors of cytokine signalling (SOCS) proteins, can also influence the activation status of the JAK/STAT signalling pathway in disease (JC Tan and Rabkin R, Pediatric Nephrology 2005, 20, 567-575).
Several mutated forms of JAK2 have been identified in a variety of disease settings. For example, translocations resulting in the fusion of the JAK2 kinase domain with an oligomerization domain, TEL-JAK2, Bcr-JAK2 and PCM1-JAK2, have been implicated in the pathogenesis of various hematologic malignancies (SD Turner and Alesander D R, Leukemia, 2006, 20, 572-582). More recently, a unique acquired mutation encoding a valine-to-phenylalanine (V617F) substitution in JAK2 was detected in a significant number of polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis patients and to a lesser extent in several other diseases. The mutant JAK2 protein is able to activate downstream signalling in the absence of cytokine stimulation, resulting in autonomous growth and/or hypersensitivity to cytokines and is believed to play a critical role in driving these diseases (M J Percy and McMullin M F, Hematological Oncology 2005, 23(3-4), 91-93).
JAKs (in particular JAK3) play an important biological roles in the immunosuppressive field and there are reports of using JAK kinase inhibitors as tools to prevent organ transplant rejections (Changelian, P. S. et al, Science, 2003, 302, 875-878). Merck (Thompson, J. E. et al Bioorg. Med. Chem. Lett. 2002, 12, 1219-1223) and Incyte (WO2005/105814) reported imidazole based JAK2/3 inhibitors with enzyme potency at single nM levels. Recent Vertex PCT publications have described azaindoles as JAK inhibitors (WO2005/95400). AstraZeneca has published quinoline-3-carboxamides as JAK3 inhibitors (WO2002/92571).
In addition to the above, Vertex Pharmaceuticals has described pyrazole compounds as inhibitors of GSK3, Aurora, etc. in WO2002/50065, WO2002/62789, WO2003/027111 and WO2004/37814; and AstraZeneca has reported pyrazole compounds as inhibitors against IGF-1 receptor kinase—WO2003/48133- and Trk in WO2005/049033, WO2005/103010, WO2006/082392.